1. Field of the Invention
This invention relates generally to instruments for the measurements of gas ion flux density, and more particularly to a probe for this purpose whose presence in the atmosphere in which a measurement is being made does not give rise to perturbations in the localized electric field and therefore has no appreciable effect on the accuracy of measurement.
2. Prior Art
In a gas such as air, ionization takes place as a result of an electric discharge through the gas which dislodges electrons from neutral gaseous atoms so that the atoms then become charged ions. Each gas has a critical voltage at which the ionization phenomenon is initiated.
A corona discharge is a highly active glow region in the atmosphere surrounding a discharge electrode. If this electrode is constituted by a wire, the glow region extends a short distance beyond the wire. Assuming that the wire is negatively charged, the free electrons in the gas in the region of the intense electric field surrounding the wire gain energy from this field to produce positive ions and other electrons by collision. In turn, these new electrons are accelerated and produce further ionization.
This cumulative process results in an electron avalanche in which the positive ions are accelerated toward and bombard the negatively-charged wire. As a consequence of such ionic bombardment, secondary electrons are ejected from the wire surface which act to maintain the discharge. Moreover high-frequency radiation originating from excited gas molecules lying within the corona region contribute to the supply of secondary electrons.
The electrons emitted from the negatively-charged wire or discharge electrode are drawn toward a positively-charged electrode. As these electrons advance into the weaker field away from the wire, they tend to form negative ions by attaching themselves to neutral oxygen molecules. These negative ions create a dense unipolar cloud that occupies most of the gap between the electrodes and constitutes the only current in the entire space outside of the corona glow region. This space charge functions to retard the further emission of negative charge from the corona region and in this way restricts the ionizing field adjacent the wire, thereby stabilizing the discharge.
The type of corona produced depends on the polarity of the discharge or ionizing electrode. In the example given above, we have assumed a negative polarity, in which case positive ions are accelerated toward the electrode and negatively-charged oxygen ions are repelled therefrom to produce a corona discharge. Conversely, when the polarity of the ionizing electrode is positive, negative ions are accelerated toward the electrode, causing the breakdown of air molecules with the result that positive ions are repelled outward from the ionizing electrode to create a corona glow.
The measurement of gaseous ions using the flux density technique has many useful practical applications. Ion flux density instruments may be used to measure the output of ionizing sources, to measure atmospheric ion flux as well as to monitor ion activity in research and development projects. Thus in searching for defects in high voltage electrical components and assemblies, ultra low current leakage can readily be detected by probing the atmosphere surrounding the items being tested for ion activity, even before the onset of corona, in order to determine the location of the ion source.
The probe conventionally used for this purpose includes a metal target of known area, the number of ions that give up their charge to this target being indicated in terms of an electrical current induced in an electrometer connected to the target. Because the target has a known area, the amount of current it receives affords an index to the ion flux density.
An electrometer is a highly sensitive instrument which is capable of measuring low-level potentials in that it has an extremely high input impedance. Before the introduction of electronic electrometers of the vacuum tube and later of the solid-state type, electrometer measurements were generally carried out by moving-coil galvanometers. For example, the solid-state ion flux meter (model 700) marketed by Santek, Inc. of Hollywood, Florida, has an extremely high input impedance and is capable of measuring currents of the order of 1.times.10.sup.-14 amperes.
In existing types of probes associated with electrometers, the metal target is generally disc-shaped and is mounted at one end of a supporting barrel fabricated of dielectric material, the target being connected to a conductor extending axially through the barrel and joined to the inner tubular connector of a jack socketed in the other end of the barrel. Connection to the input terminal of the electrometer is effected by a shielded cable whose end coupler has a plug which is inserted in the inner connector of the jack and a ring which is joined to the outer connector. The electrometer and the shield are both grounded.
The presence of a probe of this type in an atmosphere whose ion flux density is to be measured gives rise to localized electric field perturbations which introduce an error in the reading so that the instrument is lacking in accuracy. The reason for this inaccuracy will now be explained.
Air ions flow along electric field lines of force, these force lines being lines of potential difference. When a probe connected to an electrometer is placed in an atmosphere having ions therein, the probe is usually the component of lowest resistance in the circuit, for the electrometer presents an extremely high impedance to the ion source and so does the atmosphere.
The barrel of the probe is of dielectric material, and in the presence of an electrostatic field a boundary surface charge quickly builds up on the surface of the barrel which interfaces with the atmosphere. This surface charge reduces the electric field gradient between the probe and the source to a value approaching zero. When this condition is brought about, few ions will strike the target. As noted previously, air ions flow along electric lines of force created by a potential difference; and when this difference is close to zero, there is little ion flow.
This drawback cannot be overcome by making the barrel of the probe of a conductive material, for then the barrel will present a ground plane to the atmosphere. This ground plane is artificial, for it increases the voltage gradient and brings about a higher flow of ions than would occur were the probe's grounding effect removed. Hence with a conductive barrel in a flux density probe, the reading would be inaccurate.